FIG. 9 is a cross sectional view showing a conventional stacked type solid state image sensing apparatus as disclosed, for example, in Proceedings of National Conference of Television Association by Yano et al., 1985 p.63.
Referring to FIG. 9, a p type semiconductor layer 2 is provided on an n type semiconductor substrate 1. Provided on the p type semiconductor layer 2 are a p type semiconductor layer 3 in which a charge transfer portion is to be produced, and an n type semiconductor layer 5 for forming a signal charge storage portion. An n type semiconductor layer 4 forming a buried channel for charge transfer is disposed on a main surface of the p type semiconductor layer 3. A silicon oxide film 6 is provided on a main surface of the p type semiconductor layer 3. A polysilicon gate electrode 7 for reading and transferring charge is provided on the n type semiconductor layer 4 with the silicon oxide film 6 therebetween. A polysilicon layer 9 which is a part of an interconnection between a photoelectric conversion portion and the charge storage portion is connected to the n type semiconductor layer 5. An insulating layer 8 is provided on the n type semiconductor substrate 1 covering the silicon oxide film 6 and the polysilicon layer 9. A metal layer 11 for shielding light is provided in the insulating layer 8. An interconnection metal 10 that is separate for every pixel is disposed on the insulating layer 8. A photoelectric conversion film 12 formed of intrinsic amorphous silicon or the like is disposed on the n type semiconductor layer 1 so as to cover the interconnection metal 10. Provided on the photoelectric conversion film 12 is an electron injection preventive layer 22 formed of p type amorphous silicon carbide or the like. A transparent electrode 14 is provided on the electron injection preventive layer 22.
Now, an operation will be described.
With a negative voltage being applied to the transparent electrode 14, light is incident on the photoelectric conversion film 12. Among charges generated in the photoelectric conversion film 12, electrons move toward the interconnection metal 10 because of an electric field in the photoelectric conversion film 12 and are stored in the n type semiconductor layer 5. The stored charges move to the n type semiconductor layer 4 upon application of a large positive voltage to the gate electrode 7. The signal charges are transferred in one direction by the function of a vertical charge transfer element formed in the n type semiconductor layer 4, and then externally read out through a horizontal charge transfer element which is not shown.
A conventional stacked type solid state image sensing apparatus utilizing amorphous silicon carbide which is the most generally used for an electron injection preventive layer has the following problem.
FIG. 10 (a) is a representation showing an energy band in the vicinity of the interface between a photoelectric conversion film and an electron injection preventive layer when amorphous silicon carbide is utilized for the electron injection preventive layer. This figure is disclosed by Katayama et al. in Proceedings of National Conference of Television Society, 1991, p.17. In the figure, reference numeral 14 represents a transparent electrode, 22 an electron injection preventive layer, and 12 a photoelectric conversion film, while E.sub.C represents energy at the conduction band edge, E.sub.V energy at the valence band edge, and E.sub.F a Fermi level. V represents a vacuum level, .phi..sub.M a work function for the transparent electrode 14, x.sub.2 the electron affinity of amorphous silicon carbide (22), Eg.sub.2 the band gap of amorphous silicon carbide (22), x.sub.1 the electron affinity of the photoelectric conversion film 12, and Eg.sub.1 the band gap of the photoelectric conversion film 12.
As can be seen from FIG. 10 (a), when amorphous silicon carbide is used for the electron injection preventive layer, since .phi..sub.M -x.sub.2 is small (judged from a value relative to Eg.sub.1), in other words, its energy barrier is small, dark current cannot be prevented effectively.
A detailed description will be provided of generation of dark current in conjunction with FIG. 11. FIG. 11 shows an energy band when light irradiates a solid state image sensing apparatus having an energy band shown in FIG. 10 (a). In the figure, A represents a dark current component (electrons) thermally generated from the electrode 14 and injected into the photoelectric conversion film 12. B is a dark current component thermally generated in the photoelectric conversion film 12. C is a signal charge when light is incident inside the photoelectric conversion film 12. In the figure, an empty circle represents a hole, and a solid circle represents an electron. S/N ratio is generally given by the following equation: EQU S/N ratio=C/(A+B)
Since B is a fixed value, A should be reduced in order to increase the value of S/N. In the solid state image sensing apparatus having an energy band illustrated in FIG. 10 (a), since .phi..sub.M -x.sub.2 is small (its energy barrier is small), electrons (dark current) thermally generated from the transparent electrode 14 and coming into the photoelectric conversion film 12 cannot be prevented effectively. Consequently, the value of A is increased, resulting in a small S/N ratio.
Further, since x.sub.2 +Eg.sub.2 is larger than x.sub.1 +Eg.sub.1, an energy barrier 122 is generated, resulting in the accumulation of holes between the photoelectric conversion film 12 and the electron injection preventive layers 22.
As a result, the electrons and the holes recombine, the potential of the photoelectric conversion film 12 changes, degrading the performance of the apparatus.
FIG. 10 (b) is another conventional example, and shows an energy band of the target of an image sensing tube utilizing antimony trisulfide (Sb.sub.2 S.sub.3) for an electron injection preventive layer 23 in the vicinity of the interface between the photoelectric conversion film 12 and the electron injection preventive layer 23. This energy band representation is disclosed by Tanioka et al. in Television Society Papers, Vol. 44, No. 8 (1990) p. 1074. Also in this conventional example, as can be seen from FIG. 10 (b), dark current cannot be prevented because .phi..sub.M -x.sub.2 is small.
FIG. 10 (c) is an energy band representation in the vicinity of the interface between the photoelectric conversion film 12 and an injection preventive layer 24 when an amorphous silicon nitride film is used for the electron injection preventive layer 24. The energy band representation given in FIG. 10 (c) is disclosed by Hatanaka et al. in Electronic Information Communication Society Technical Report SDM87-15 (1987), p. 19. As can be clearly seen from the figure, dark current cannot be prevented because the value of .phi..sub.M -x.sub.2 is small. Furthermore, according to this conventional example, holes are accumulated in the interface between the photoelectric conversion film 24 and the electron injection preventive layer 12.
It is therefore an object of the invention to provide an improved photoelectric converter with reduced dark current.
Another object of the invention is to provide an improved photoelectric converter preventing holes from being accumulated between a photoelectric conversion film and an electron injection preventive layer.
A further object of the invention is to provide an improved solid state image sensing apparatus with reduced dark current.
Yet another object of the invention is to provide an improved image sensing tube target with reduced dark current.
A still further object of the invention is to provide an improved solar battery with reduced dark current.
A still further object of the invention is to provide an improved avalanche photodiode with reduced dark current.
In order to achieve the above-stated objects, a photoelectric converter in accordance with the invention includes a substrate, a first electrode on the substrate, and a semiconductor on the substrate covering the first electrode for photoelectric conversion. A second electrode is on the semiconductor. An electron injection preventive layer for preventing electrons from being injected from the second electrode to the semiconductor is inserted between the semiconductor and the second electrode. The electron injection preventive layer is formed of a material satisfying the following inequality, where the work function of the second electrode is .phi..sub.M, the electron affinity of the electron injection preventive layer is x.sub.2, and the band gap of the semiconductor is Eg.sub.1 : EQU .phi..sub.M -x.sub.2 .gtoreq.Eg.sub.1
In a photoelectric converter in accordance with another aspect of the invention, an electron injection preventive layer is formed of a material further satisfying the following inequality, where the electron affinity of the electron injection preventive layer is x.sub.2, and the electron affinity of the semiconductor is x.sub.1 : EQU x.sub.2 .ltoreq.x.sub.1
In a photoelectric converter in accordance with another aspect of the invention, an electron injection preventive layer is formed of a material further satisfying the following inequality, where the band gap of the electron injection preventive layer is Eg.sub.2, the electron affinity of the semiconductor is x.sub.1, and the band gap of the semiconductor is Eg.sub.1 : EQU x.sub.2 +Eg.sub.2 .ltoreq.x.sub.1 +Eg.sub.1
A photoelectric converter in accordance with yet another aspect of the invention is directed to a solid state image sensing apparatus including the above-stated properties.
A photoelectric converter in accordance with a still further aspect of the invention is directed to an image sensing tube target including the above-stated properties.
A photoelectric converter in accordance with a yet another aspect of the invention is directed to a solar battery including the above-stated properties.
A photoelectric converter in accordance with a still further aspect of the invention is directed to an avalanche photodiode including the above-stated properties.
In a photoelectric converter according to the invention, since an electron injection preventive layer is formed of a material satisfying the following inequality where the work function of the second electrode is .phi..sub.M, the electron affinity of the electron injection preventive layer is x.sub.2, and the band gap of semiconductor is E.sub.1, a dark current component thermally generated in the second electrode is interrupted by a high energy barrier and will not enter the photoelectric conversion film. EQU .phi..sub.M -x.sub.2 .gtoreq.Eg.sub.1
In a photoelectric converter in accordance with another aspect of the invention, since the above-stated electron injection preventive layer is formed of a material further satisfying the following inequality, where the affinity of the electron injection preventive layer is x.sub.2, and the electron affinity of the semiconductor is x.sub.1, not only the dark current component can be prevented from entering the photoelectric conversion film from the second electrode, but also holes generated in the photoelectric conversion film will not be accumulated in the interface between the photoelectric conversion layer and the electron injection preventive layer. EQU x.sub.2 .ltoreq.x.sub.1
In a photoelectric converter in accordance with yet another aspect of the invention, since the above-stated electron injection preventive layer is formed of a material satisfying the following inequality where the electron affinity of the electron injection preventive layer is x.sub.2, the band gap of the electron injection preventive layer is Eg.sub.2, the electron affinity of the semiconductor is x.sub.1, and the band gap of the semiconductor is Eg.sub.1, holes will not be accumulated in the interface between the photoelectric conversion layer and the electron injection preventive layer. EQU x.sub.2 +Eg.sub.2 .gtoreq.x.sub.1 +Eg.sub.1
In a photoelectric converter in accordance with yet another aspect of the invention, a photoelectric converter having the above-stated properties is applied to a solid state image sensing apparatus, so that an improved solid state image sensing apparatus with reduced dark current can be provided.
In a photoelectric converter in accordance with a still further aspect of the invention, since a photoelectric converter having the above-stated properties is applied to an image sensing tube target, an image sensing tube target with reduced dark current can be provided.
In a photoelectric converter in accordance with yet another aspect of the invention, since a photoelectric converter having the above-stated properties is applied to a solar battery, an improved solar battery with reduced dark current can be provided.
In a photoelectric converter in accordance with a still further aspect of the invention, since the above-stated photoelectric converter is applied to an avalanche photodiode, an avalanche photodiode with reduced dark current can be provided.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.